Sequence Diagram

• Class Diagrams and Object Diagrams present static information
• in a functioning system however objects interact with each other over time
• A UML Sequence Diagram is used to show the details od these object interactions over time
• Consider a washing machine system which consists of a number of objects including a water pipe, washing machine, drum and drain
• What is the sequence of a actions performed once the clothes are added and a wash cycke started

• Use Cases suggest how actors interact with the system that will ultimately to implemented in software
• During this interaction, an actor will generate events to the system, requesting a particular operation in response
• It is often desirable to isolate and document the operations that an actor requests of a system
• A system sequence diagram is a drawing that shows for a particular use case scenario, the events, external actors generate their order and inter system events all against time.

Representing objects

• squares with object type optionally proceeded by object name and colon
• write object's name if it clarifies the diagram
• object's "life line: represented by dashed vertical line

Messages between objects

• message indicated by horizontal arrow to other objects
• write message name and arguments above arrow
• dashed arrow back indicates return messages

Lifetime of object

• creation: arrow with new written above it
• notice that an object created after the start of the scenario appears lower than the other
• deletion an X at bottom of object's life line
• Java does not explicitly delete objects, they fall out of scope and are garbage collected

Indicating method calls

• activation: think box over object's life line drawn when object's method is on the stack
• either that object is running its code, or it is on the stack waiting for another object's method to finish